This invention concerns remediation of surface waters by removal of toxic metals prior to discharge of the waters to streams and lakes. In particular the invention concerns overburden from surface coal mining in certain regions where selenium-containing rock is present in the overburden, and where rainwater leaching through the rock introduces objectionable levels of selenium into the runoff water, typically in the form of selenate, these levels being too high for discharge to streams and lakes.
The subject of this invention is related to that of U.S. Pat. Nos. 4,940,549, 5,200,082, 5,575,919, 5,866,014, 6,093,328 and 6,926,878. In particular, the latter four patents, U.S. Pat. Nos. 5,575,919, 5,866,014, 6,093,328 and 6,926,878 disclose preparation of a sulfur-modified iron premix (SMI) for use in treating water to remove certain substances. This invention utilizes a sulfur-modified iron (SMI) premix similar to that produced in accordance with the above patents, in a reaction column to treat runoff water laden with selenium, in particular, selenate. All of the above patents are incorporated herein by reference.
Surface coal mining operations produce large volumes of overburden, which is removed to reach coal seams. In the United States the overburden must ultimately be replaced when the mining operation is completed or moves on, and the land is reclaimed essentially back to its original condition and topography. However, the overburden exposes a multitude of rocks to rainfall, i.e. to moving water which can leach the rocks and take on substances, including minerals, present in the rocks. Rainwater is nearly pure and is a strong leaching agent. These rocks if left undisturbed would be subject to little or no rainwater leaching. The result is that runoff from rain can carry objectionable levels of dissolved substances, leached from the exposed rock, into lakes and streams.
Particularly in the Appalachian region of the United States many surface coal mining locations have rock and soil (“rock” herein) that contains selenium, especially in the selenate form. The removal of the overburden exposes these rocks and even after reclamation, runoff from rainwater in many cases exceeds permissible levels of selenium allowed for discharge to lakes and streams. In many cases the runoff water, or much of it, has been trapped in detention “ponds” at various locations around the site of a surface mine, including after reclamation, pursuant to regulations that prohibit sediment-laden runoff water from being discharged to lakes and streams at excavation sites. However, eventually the pond water must be withdrawn or allowed to overflow, ultimately reaching lakes or streams. Although sediment is much less, dissolved minerals remain, including selenium. There has been no economically feasible method for remediation of this selenium problem, and it affects many surface mines, particular in the Appalachian region, as well as others.